For the BLM to cause a second wave it would need for there to be quite a number of infected people out and about in order to infect people. With testing having been freely available for all with any symptom for some time now I very much doubt that is the baseload of infected people to create a second wave.
It may well cause a second ripple though, but I believe it would much more probably would be a hotspot at worst. So yes lets hope that there are not repeated such protests as the greater mixing could multiply cases.
Not impossible, but yes will be incomplete. For mass gatherings like this, and things such as using public transport, is why the Covid Safe App was deployed and is important. However this one case did not have it on. If people are going to gather ANYWHERE they should have the app installed and on. Attending a rally for the common good, but not having the app on for common good indicates a lack of genuineness to me.
With the BLM protester all their close contacts, including those that they attended the rally with, have been put into compulsory self-isolation.
PS I personally was not in favour of the protest, but it was always going to happen.